Wire and cable insulation are normally quite flammable, unless they are made from costly inherently flame retardant materials. As a result, they can poses a fire propagation hazard in power plants, distribution areas, manholes, and buildings. Ignition can easily occur from overheating or arcing. Non-halogenated wire and cable insulation typically consist of polyolefins (polyethylene, polypropylene and/or copolymers thereof) or copolymers of olefins with vinyl monomers, such as vinyl acetate or ethyl acrylate. These polymers have the burning characteristics of saturated hydrocarbons with high heats of combustion and a tendency to burn completely, without forming char. The polyolefins and olefin copolymers, absent the inclusion of halogen additives, are especially difficult to flame-retard.
Presently, flame-retarding of wire and cable insulation is accomplished in three principal ways. One approach is to use halogenated additives: for example, chlorinated polymers such as chlorosulfonated polyethylene, neoprene, polyvinyl chloride, or the like. Halogen-containing additives also can be used, such as decabromodiphenyl oxide, sold under the trademark Dechlorane Plus.TM., tetrabromophthalimides, chlorowaxes, or the like. Frequently, these halogenated polymers or additives are boosted or "synergized" in their flame retardant activity by adding antimony oxide. The inherent shortcoming of using halogenated materials and halogen containing additives is that the gases evolved (i.e. hydrogen chloride or hydrogen bromide) during burning, or even merely overheating, are corrosive as well as being highly irritating to the eyes and respiratory system.
A more recent basis of concern regarding halogenated flame retardants is that they may pose an environmental hazard, because they can persistent in the environment and are capable of combustion or pyrolysis under certain conditions to form toxic compounds, such as polyhalodibenzodioxins or polyhalodibenzofurans. Whether or not they constitute an actual hazard, the use of halogenated materials has raised environmental concerns and they are now the subject of increasing regulatory attention.
A second approach to providing flame retardancy for wire and cable insulation, especially those made of polyolefins and olefin copolymers, is to use a hydrated mineral such as alumina trihydrate or magnesium hydroxide. By using high loadings, such minerals provide an endothermic water release under heating and burning conditions that effects a flame retardant action. The drawback of these systems is the need for high loadings, which render the insulation undesirably stiff and abrasive.
There are processing difficulties in manufacturing highly loaded hydrated mineral polymer systems as well. Some improvement can be achieved with coupling agents, such as silanes and silicones, but at substantial cost. Furthermore, these hydrated minerals are polar in character and exert quite adverse effects on the electrical resistance and dielectric characteristics of the insulation. Cable insulation containing hydrated minerals are only marginally useful for voltages above about 2-3 kV, and are generally unsatisfactory for use as primary insulation.
A third approach employs phosphorus compounds, as well as char-forming and intumescent additives. Typical formulations frequently applied to the flame-retarding of polyolefins and olefin copolymers use ammonium polyphosphate as the char-forming catalysts and a resin, such as dipentaerythritol resin, ethyleneurea-formaldehyde resin or triazinepiperazine resin as the source of the char. Such systems are expensive and have several electrical resistance and dielectric shortcomings. They can also not be used in high voltage or primary insulation applications. Ammonium polyphosphate component is hydrolyzable and forms electrically conductive water-soluble ammonium phosphate. The use of aryl or alkyl phosphate esters causes exudation and, as a result, they are relatively inefficient flame retardants in polyolefins.
In view of the short-comings of the commonly used approaches, there is a need for flame retardant systems for normally flammable insulation, in particular polyolefins and olefin copolymers, that does not cause corrosive or toxic gas emissions, or unduly stiffen or otherwise adversely affect the physical properties of the insulation, and which permits retention of good electrical properties.
The use of polyphenylene oxides in miscible blends with styrenic polymers is well known in the art. Due to their compatible solubility parameters, polyphenylene oxides even form true alloys with, for example, high impact polystyrenes. Such polyphenylene oxide-styrenic polymer blends have been flame retarded by many means, including the use of various phosphorus additives. Melamine has been used in some of these flame retardant systems.
It appears that blends of polyphenylene oxides (ethers) with polyolefins have not been widely used because, unlike styrenic copolymers, polyolefins do not form true polymer solutions with polyphenylene oxides. The use of polyphenylene oxides in polyolefins is disclosed by Abolins et al. in Canadian Patent No. 1 253 283. To achieve flame retardancy, Abolins et al. discloses the addition of at least an organic phosphate ester as a requirement and optionally a halogenated additive to the resin. The use of melamine and/or an effective amount of a silicaceous mineral as part of the flame retardant system is absent.
It is an object of the present invention to provide flame retardant blends for polyolefins and olefin copolymers.
It is a further object to provide flame retardant insulation formulations that produce little or no corrosive smoke.